Continuous diffuser washers have been extremely successful in simplifying pulp washing operations, especially immediately after continuous digestion of the pulp. Conventional diffuser washers are usually mounted on the brown stock storage tank and effect washing in a closed system where no air comes in contact with the pulp, reducing the tendency to foam, facilitating pollution abatement, and permitting greater recycling and reuse of liquors. The pulp is passed upwardly in the diffuser vessel and passes between a plurality of concentric withdrawal screen rings, washing liquid being introduced through tubes that are rotating within the areas defined by the screen rings and being attached to a scraper at the top of the vessel. The screens are moved up and down by hydraulic cylinders mounted outside the vessel walls, suitable mechanical interconnections extending through the vessel walls from the cylinders to the screens, and also providing for withdrawal of liquid from the screens through the vessel walls. Such a reciprocating screen arrangement is also useful for thickening of pulp, either in a separate thickening vessel or as part of the diffusion washing sequence. Exemplary prior art diffuser washers and/or thickeners are shown in U.S. Pat. Nos. 3,348,390; 3,372,087; 4,076,623; and 4,100,069.
While conventional continuous washers are successful in performing their intended functions, and provide the most efficient commercially available washing to date, there have been a number of practical problems associated therewith. For instance, because of the ring-like construction, the components are expensive to fabricate, and each screen ring and liquid-introducing nozzle within a given structure must be constructed differently in order to provide varying flow rates to accomplish uniform washing. The ring construction also means that significant bending and twisting stresses are applied to the structure as it is reciprocated up and down, especially providing stress at the connections of the arms to the rings and sometimes resulting in premature failure of the assembly. The relative movement between the diffuser tubes and the screens causes a shock wave in the area between the tubes and screens. The structures for packing the central rotor, and for packing and sealing the arms where they extend through the vessel side walls to the cylinders, are expensive and require considerable maintenance if leakage is to be prevented. Also, rotation of the central rotor results in significant energy usage.
According to the present invention, any problems inherent in prior art continuous diffuser washers are eliminated by providing a greatly simiplified structure. The structure according to the present invention has fewer moving parts, has less expensive parts that are easier to fabricate, does not require the vessel side walls to be penetrated at all, and in general provides a more simplified and efficient structure for effecting diffusion washing, bleaching and/or thickening.
According to one aspect of the present invention, an upright hollow vessel is provided containing a plurality of withdrawal screens extending substantially parallel to the vessel direction of elongation, conduit means for supporting the withdrawal screens and for providing passage of fluid from the withdrawal screens to an area remote from the withdrawal screens, and means for reciprocating the conduit means with attached withdrawal screens up and down in a direction substantially coincident with the direction of elongation of the vessel. The reciprocating means consist essentially of a single linear actuator located within the cross-sectional area of the vessel (and preferably above the vessel), the provision of such reciprocating means allowing the conduit means and withdrawal screens to be constructed so that no structures penetrate the vessel walls. All fluid withdrawal, and fluid introduction, comes from above or below the vessel, preferably from above.
According to the present invention, fluid introducing structures are also provided supported by said conduit means, the conduit means providing for passage of fluid from an area remote from the fluid introducing structures to the fluid introducing structures. The withdrawal screens and fluid introducing structures are interspersed with each other and are stationary with respect to each other. This arrangement allows the construction of the withdrawal screens and fluid introcing structures in a wide variety of manners. For instance, instead of being provided as rings, the screens and structures can be disposed in concentric quadrates (when viewed along the vessel direction of elongation), or in parallel straight lines. All of the screens and fluid introducing structures when in a given vessel can thus be constructed in the same manner, and the withdrawal screens and fluid introducing structures can be made interchangeable with each other. The fluid introducing structures are preferably formed as a plurality of plates having openings formed therein that are larger and more widely spaced than openings provided in the withdrawal screens, the openings being prismoid in configuration and having a larger cross-sectional area closer to the fluid introducing structure end than the center.
In order to effect removal of the upward flowing pulp from the vessel, since the central rotating scrapers of the prior art structures have been eliminated some alternative structure must be provided. The alternative structure preferably consists essentially of a plurality of distinct wiper blades elongated in a direction perpendicular to the direction of elongation of the vessel, and means for oscillating the wiper blades about axes parallel to the direction of elongation of the vessel. The oscillating means are preferably disposed above the vessel and within the cross-sectional area thereof, a single linear actuator being provided for actuation of all of the wiper blades at the same time. The vessel may be a polygon in cross-section with withdrawal conduits provided along each side of the vessel, and with a wiper blade mounted at each corner of the vessel at the top thereof. Where the vessel is quadrate in cross-section, each wiper blade has an effective length of slightly more than one-half the length of the vessel sides merging to form the corner at which the blade is disposed.
The linear actuator for reciprocating the conduit means with attached withdrawal screens up and down preferably moves the structures upwardly at a first rate a predetermined distance, and then downwardly at a second rate the predetermined distance, the second rate being much greater than the first rate and great enough to facilitate dislodgement of materials clinging to faces of the withdrawal screens when in use for treating pulp. This action is described in U.S. Pat. Nos. 3,348,390 and 3,372,087. Alternatively, the structures could be continuously oscillated utilizing a mechanism such as described in U.S. Pat. No. 4,076,623. The apparatus according to the present invention is best suited for the washing and/or thickening of cellolusic pulp material having a consistency of about 6-15% (8-12% being the approximate maximum efficiency range), and performs its intended functions in a simple and efficient manner.
It is the primary object of the present invention to provide an improved apparatus and method especially adapted for washing, bleaching, and/or thickening of pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.